ND Filters and Shutter Speed:

ND Filters and how to get the most from your long exposures:

The benefits of using an ND filter to reduce your depth of field are well known. On bright days, depending on your artistic intent ND filters are almost a necessity. However, the other main advantage of using an ND filter is for long exposures. For example if you want to blur the motion of water or clouds to give a nice smooth effect or make moving objects such as people or cars invisible in an image where your image for example is a busy street and you want to capture the motion yet not the clutter. This trick is also quite useful when you want to capture busy tourist locations. The use of ND filters in this case can make a normally busy and chaotic scene look somewhat surreal.

So how do you go about using an ND filter for long exposures? Depending on your aim there are ND filters to suit all types of long exposures and the correct filter will depend on the exposure time, the lighting conditions and your other camera settings, aperture and ISO. One of the first things to take into consideration is the diffraction limit of your lens. All lenses have this limit and it is important you stay below it for reasons of sharpness. So as a start, you should set your aperture to around the f/8 to f/11 mark as most lenses are at their sharpest around this region. You should also set your ISO to the base ISO of the camera itself and not the extended low ISO reading. Setting your camera’s ISO to the extended low will reduce the dynamic range of your sensor so it is best to simply stay at the sensor base ISO which for most DSLR cameras out there is 100 or 200.

ND Neutral Density Filters and Shutter SpeedSo that is your aperture and ISO sorted. Now all that you need concentrate on is your shutter speed. Enter ND filters. The first thing to determine is what exposure time you need. The main consideration here is the type of artistic effect you are looking for. Take an example using a set of extreme conditions. The location is a beach on the afternoon of a sunny day. You want to get a nice blurred photo of waves lapping up against some rocks . Using an adapted version of the Sunny 16 rule you end up with settings f/11, ISO100, 1/250s for good exposure (Sunny 16 rule settings are by default f/16, ISO100, 1/125s shutter speed). At this point you have max sharpness (aperture), max dynamic range (ISO) and least noise (ISO). However a 1/250s exposure time is not much use for those silky water effects you require. You will need a shutter speed of 2 seconds or more for any sort of smooth blur effect. What sort of ND filter would we need to mount on the lens to go from 1/250s to 2 seconds? We can find out by calculating how many stops (EV’s or f-stops) there are between the two exposure times:

F-stop Difference 0 1 2 3 4 5 6 7 8 9 10
Exposure Time 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s 2s 4s

From the above table we need a 9 stop ND filter to give us 2 seconds of blurriness! Or for an even better blurred effect you could go to 4 seconds with a 10 stop or 1000x (10 f-stop / optical density 3.0) ND filter. So, you will need a pretty heavy duty ND filter for midday, beach-based long exposures.

In terms of long exposures, because each filter manufacturer labels them differently it is important that you know exactly what each labelling type means. Hoya filters use the ND32x marking. What this means is that using an ND32x ND filter will allow you use shutter speeds that are 32 times longer. Looking at the table above and using our previous example of the 1000x ND filter. You can see that to get from 1/250s to a 4 second exposure we need a 10 stop filter. This is labelled by some brands as an ND1000x filter. So I can get an exposure time that is 1000 times longer with this 10 f-stop filter. And in our example, 1000 times our original exposure time of 1/250s is in fact 4 seconds. If the filter is a Heliopan for example it will be labelled using optical density (for example 0.9, 1.5, 2.7 etc) and in this case you simply divide the number by 0.3. The result will tell you many f-stops the filter provides. The table below details the filters according to their brand markings.

Optical Density 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3.0
Exposure Factor 2x 4x 8x 16x 32x 64x 125x 250x 500x 1000x
F-Stop Reduction 1 2 3 4 5 6 7 8 9 10

So, now that the ND filter branding riddle is solved, what sort of exposure times can you hope to get using them. Well this all depends on your starting exposure time. So in our example earlier, the starting shutter speed was 1/250s and we needed a 10 stop (1000x) ND filter to give us a 4 second exposure time. The table below gives you an idea of what filter you need for a certain exposure time based on a non-filtered, correctly exposed initial shutter speed.

Unfiltered Starting Shutter Speed (correctly exposed) 0.3 O.D
ND2x
f-stop 1
0.6 O.D
ND4x
f-stop 2
0.9 O.D
ND8x
f-stop 3
1.2 O.D
ND16x
f-stop 4
1.5 O.D
ND32x
f-stop 5
1.8 O.D
ND64x
f-stop 6
2.1 O.D
ND128x
f-stop 7
2.4 O.D
ND256x
f-stop 8
2.7 O.D
ND512x
f-stop 9
3.0 O.D
ND1000x
f-stop 10
1/8000s 1/4000s 1/2000s 1/1000s 1/500s 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s
1/4000s 1/2000s 1/1000s 1/500s 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s
1/2000s 1/1000s 1/500s 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s
1/1000s 1/500s 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s
1/500s 1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s 2s
1/250s 1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s 2s 4s
1/125s 1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s 2s 4s 8s
1/60s 1/30s 1/15s 1/8s 1/4s 1/2s 1s 2s 4s 8s 15s
1/30s 1/15s 1/8s 1/4s 1/2s 1s 2s 4s 8s 15s 30s
1/15s 1/8s 1/4s 1/2s 1s 2s 4s 8s 15s 30s 1 min
1/8s 1/4s 1/2s 1s 2s 4s 8s 15s 30s 1 min 2 mins
1/4s 1/2s 1s 2s 4s 8s 15s 30s 1 min 2 mins 4 mins
1/2s 1s 2s 4s 8s 15s 30s 1 min 2 mins 4 mins 8 mins
1s 2s 4s 8s 15s 30s 1 min 2 mins 4 mins 8 mins 15 mins
2s 4s 8s 15s 20s 1 min 2 mins 4 mins 8 mins 15 mins 30 mins
4s 8s 15s 30s 1 min 2 mins 4 mins 8 mins 15 mins 30 mins 1 hour
8s 15s 30s 1 min 2 mins 4 mins 8 mins 15 mins 30 mins 1 hour 2 hours

Of course you don’t need to go out and buy an ND256x filter if you already have say an ND32x and an ND8x. Combining both of these filters on your lens will give you the same effect as an ND256x. In other words the effect is cumulative. Your 5 f-stop ND32x filter plus your 3 f-stop ND8x filter will, if combined give you 8 stops in total. Alternatively, 8 (for ND8x) multiplied by 32 (for ND32x) = 256 which is your ND256x, 8 f-stop ND filter. It’s all very complicated isn’t it! I blame the different manufacturers! There is only one small thing to note if you are combining ND filters on your lens. It can lead to vignettes. This is very much the case when you are using ultra wide angle lenses as the angle of view is so extreme and the metal rim of the filter is sure to narrow this angle of view if you are at the smallest focal length.

ND Neutral Density Filters and Shutter Speed So, what sort of filter is the best to buy? Well, as you have seen there are just so many variables that it is impossible to provide one answer for that. Your starting shutter speed, the desired result and the camera itself all vary. What some photographers buy is what they call a variable ND filter. By twisting the filter you can change the power of the filter. These are quite expensive though and in my opinion are overpriced given they are essentially only two polarising filters joined together. Personally, I use two filters combined. I have an ND8x and an ND64x that I combine to give me a 9 f-stop ND512x filter. I do it this way for a number of reasons. The first is it is a cheap way of getting a lot of f-stops but more importantly it gives me some form of flexibility because the f-stops are evenly spaced: ND8x (3 f-stop), ND64x (6 f-stop), ND512x (9 f-stop). I am not massively concerned about the vignettes as I never shoot at ultra wide angles because most ultra wide angle lenses tend to lose sharpness in the corners at such extreme focal lengths. However the most important reason of all for choosing these two filters is that I can use them for reasons other than long exposure photography. I can use my ND8x for portrait shoots where I want to reduce my depth of field. If the conditions are extreme and I want to get really soft portraits on a beach using an f/1.4 prime then I can use my ND64x. For me it is all about flexibility and these two filters give me that.

For those with iPhones and Android devices, you can download an app called NDCalc from the app store and it will work out your correct long exposure time based on your non-filtered shutter speed and the filter type. It’s a handy little tool and saves some time when trying to calculate the correct time. Otherwise you can just print the above table and stick it in your camera bag.

ND Neutral Density Filters and Shutter SpeedWhen it comes to long exposure photography, the limit as to what you can do is only limited by your creativity. There are just so many things that can be done. And it doesn't just involve clouds, rivers, lights, people etc. Light itself can be used creatively. The two techniques are called light painting and light drawing. Light drawing is a method whereby you use a lightsource to "draw" light into your composition. In this case the light source is visible. Light painting on the other hand is used to light elements of your composition and the light itself is not visible in the frame. Take a look at the photo to the right. This uses light painting on the foreground and the long exposure will ensure enough light to expose the stars in the sky (albeit faintly in light polluted areas). Moving out into dark sky areas outside cities you can get some incredible shots of galaxies, nebulae and other such stuff in the night sky.

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